Due to diminishing quantities of coal, petroleum, and natural gas products, attention is being directed to other energy sources, including oil shale, solar energy, and nuclear energy. One source which is receiving considerable attention is biomass materials such as wood and its byproducts. This is somewhat ironic since the original source for energy in the United States and the rest of the world was wood. In fact, U.S. Pat. No. 43,112 issued on June 14, 1864 was directed to combining sawdust, tar, wood cuttings or chippings, water, and coal-tar to form an artificial fuel.
Recently, much attention has been directed to preparing briquets from wood waste. For example, L. H. Reineke wrote a U.S. Forest Research Note entitled "Briquets from Wood Residue", in November, 1964 describing various techniques available for briquetting wood residue. In addition, U.S. Pat. Nos. 3,227,530, 3,635,684, 3,843,336, 4,015,951 and 4,043,764 describe techniques for briquetting cellulosic material.
Use of available pelletized wood waste as a fuel source has achieved only limited acceptance to date. One reason for this is the relatively low heating value of pelletized wood as compared to coal. Pelletized wood can have a heating value of less than 7,000 BTU's per pound, while coal generally has heating value in excess of 9,000 BTU's per pound. Therefore, the transportation and handling costs associated with available pelletized wood are higher per BTU than for coal.
Other problems with use of available pelletized wood as a fuel source is that it has a slow burning rate and it exhibits imcomplete burnout, resulting in formation of carbonaceous residues and low combustion efficiency. In addition, pelletized wood can be harder to ignite than coal and pelletized wood can be more fragile than coal, requiring special handling to avoid crumbling and to prevent weathering. To overcome the crumbling and weathering problems, inorganic binders such as cement and silicate of soda, and organic binders such as tar, pitch, rosin, glues, and fibers have been included in the pellets. However, no binder has been found which solves the above problems, and which also is inexpensive and does not reduce the heating value of the wood.
It has been attempted to use the self-binding characteristics of various species of wood due to lignin present to avoid the crumbling problem. This can be effected with some species of wood, but not all species, by heating the wood above its minimum plastic temperature of 325.degree. F. as reported by Reineke in the above-mentioned U.S. Forest Service Research Note, and also as reported by Gunnerman in the above-mentioned U.S. Pat. No. 4,015,951. However, such high temperatures can severely limit the operating life of the pelletizing equipment and drive high BTU volatile components from the wood.
Therefore, there is a need for a fuel pellet which resists crumbling, is easily ignitable, burns fast and completely, and has a heating value approaching that of coal; and there is also a need for a method for preparing the fuel pellet which does not require high pelletizing temperature.